Photosurface



March 31, 1959 R. G. sTouDENHElMER 2,880,344

PHoTosURFAcE Filed March 2, 1951 f77/Wgl zafmfwnwr/ minfin/wwf.:

-ff' 4% @W United States Patent() ice PHoTosURFAcE Richard George Stoudenheimer, Lancaster, Pa., assignor to Radio vCorporation of America, a corporation of Delaware Application March 2, 1951, Serial No. 213,653

7 Claims. (Cl. 313-102) This invention relates to a photosurface, and more specifically to a photoemissive cathode for use in phototubes, high vacuum photomultiplier tubes, or for pickup camera tubes for television.

Antimony lms sensitized with cesium have been used as photocathodes in photomultiplier tubes. A semitransparent cesiated antimony photocathode has a spectral response over a range from 3000 A. to 6400 A. The response of the material is peaked around 4800 A. in the blue with little response in the red. Although this photosurface has fair sensitivity, it is desirable to increase its sensitivity as well as to provide a spectral response peaked more in the yellow and red regions of the spectrum. Such a spectral response would broaden the use of the photosurface in applications requiring such a response, such as pickup tubes having a red response for color television or for flying-spot pickup systems using red luminescing phosphors.

It is thus an object of the invention to provide a photosensitive device having a cathode of improved sensitivity.

It is another object of the invention to provide an antimony and cesium cathode of improved sensitivity.

An additional object of the invention is to provide a photosensitive device having a photocathode formed from antimony and cesium and having improved sensitivity to red light.

The foregoing and related objects are achieved in accordance with the invention by depositing the antirnony and cesium layer on a semi-transparent metal oxide lm. The oxide film is formed from a metal from the group consisting of iron, cobalt, nickel and chromium. The sensitivity of such a photosurface is greater than the antimony-cesium surfaces of the prior art and its spectral response is shifted toward the red to a controllable degree determined by the amount of oxidation of the metal lm.

The invention is described in greater detail in connection with the accompanying drawing, wherein:

Figure 1 is a sectional view of a photoemissive tube having a photocathode formed in accordance with the invention,

Figure 2 is a curve showing the relative sensitivity and spectral response of different photocathodes, formed in accordance with the invention.

The invention is disclosed in Figure l, as applied to a photomultiplier tube of standard form and dimensions. Such a tube is used in applications involving low-level, large-area light sources, such as scintillation counters for the detection and measurement of nuclear radiation. The tube comprises essentially, a glass envelope 10, closed at one end with a transverse wall section 12, upon which is formed a transparent photocathode surface 14. In one tube of this type, end-wall portion 12 has a diameter of approximately two inches. The exposed portion of the photocathode film 14 is approximately 11/2 inches. This provides a useful, a large, `substantially at cathode Patented Mar. 31, 1959 area which permits good optical coupling between the photocathode and a phosphor surface, used in scintillation counters, for example.

Spaced from the photocathode and along the tube axis is an accelerating electrode 16 formed as a disc and having an aperture 17 at its center. A metallic Wall coating 18 is formed on the inner surface of the tube envelope and extends from the photocathode ilm 14 axially down the tube to a portion below the accelerating electrode 16. Wall coating 18 provides electrical contact between the photocathode 14 and a lead 20, connecting the photocathode and the metallic lm 18 to a source of ground potential, as shown.

As indicated in Figure 1, a potential diiference of volts is maintained as an optimum value between accelerating electrode 16 and the photocathode 14. Photoelectrons emitted from cathode 14 are thus accelerated toward electrode 16. Wall coating 18 aids in directing the photoelectrons toward the opening 17 at the center of electrode 16. Also the shape of the envelope end-wall portion 12 tends to focus the photoelectrons into opening 17.

Photoelectrons passing through opening 17 are collected by an electron multiplier 22, which consists of a plurality of dynode electrodes enclosed in a cylindrical metal shield 24. The photoelectrons will impinge upon a first dynode electrode 26 and will initiate secondary emission therefrom having a ratio greater than unity. This secondary emission is accelerated and directed by a xed electrostatic field along curved paths to a second dynode electrode 28, and from there to a third dynode electrode 30. In like manner, the secondary electrons from dynode 30 are directed onto successive dynodes, 32, 34, 36, 38, 40, 42 and 44. Each dynode provides an amplication of the electrons striking it to form an ever-increasing stream of electrons until those emitted by the last dynode 44 are collected by an anode electrode 46. Anode 46 consists of a grid so that electrons from dynode 42 will pass therethrough to the final dynode stage 44 before collection. This type of electron multiplier is fully described in U.S. Patent 2,285,126, of Rajchman et al. The specied details of the tube and multiplier do not constitute my invention. The current collected by anode electrode 46 constitutes the current utilized in the output circuit of the tube.

Opening 17 into the multiplier section 22 is covered by a mesh grid 48. This grid is connected electrically to the rst dynode 26 and directs the secondary electrons from the dynode toward the second dynode 28. The grid also tends to prevent secondary electrons, from dynode 26, from passing back toward the photocathode 14. The rst dynode electrode 26 is xed to the accelerating disc 16 and is, thus, tied electrically to it. In normal tube operation, a potential diiference of about 75 volts is maintained between each of the succeeding dynode stages.

Commercial tubes, of the type described, have been made with a photosurface formed by putting down on the glass end wall 12, first, a semitransparent lm of antimony, and then a lilm of cesium. The photosurface is activated by baking the tube around C. This type of photosurface has provided a spectral response between 4000 A, and 6500 A, with little red response. Furthermore, the sensitivity of such a photosurface in commercial tubes is relatively low, seldom exceeding 40 microamperes per lumen, from a tungsten lamp source at a filament color temperature of 2870 K. This photosurface is primarily sensitive to blue light. In some applications it has been desirable to broaden the spectral response of the photosurface particularly y for use with light sources having emission in the yel,

Vnoise ratio.

In accordance with the invention, it has been found that, if a metal oxide film is formed on the glass end wall 12 ofthe tube, prior to the deposition of the antimony film, not only will the sensitivity of the tube be greatly increased, but also the spectral response of the tube will be shifted toward the red end of the spectrum. Furthermore, the amount of the shift can be controlled by the degree of oxidation of the metal film. Specifically, it has been found that the use of an oxide film of one of the metals selected from the group consisting of nickel, iron, cobalt and chromium, greatly increases the sensitivity of the antimony cesium photosurface, Vand shifts the peak sensitivity to longer wave lengths. Sensitivity ,as high as 80 microamperes per lumen, to tungsten light at 2870 K., have been obtained with this new photosurface.

The metal oxide film may be formed in several different ways on the surface of the glass end wall 12. However, evaporation of the metal in vacuum followed by oxidation of the metal film has proved to be the most satisfactory method.

The tube of Figure 1 is formed by mounting the electrode structure, consisting of accelerating electrode 16 and the multiplier section 22, on a plurality of lead pins 49, 51, 53, etc., sealed through a flared glass press portion 52. The press 52 consists of a reentrant portion 54 and an exhaust tubulation 56. The electrode structure is mounted on the press 52 by welding short leads from the electrodes to the support pins, respectively. The envelope portion is first coated with the conducting lilm 18 by evaporating a material, such as aluminum, in a vacuum, to make electrical contact with the cathode. During the evaporating of the aluminum, a mask or shield is placed over the end wall 12 to prevent the deposition of aluminum on the glass of the end wall, after which the mask is removed. The glass press portion 52, with the electrode structures mounted thereon, is then inserted into the open end of envelope 10 and skirt portion 54 is sealed to the end of the envelope portion 10.

The novel photosurface 14 is made in accordance with the following procedure. The method involves the use of nickel in the underlying layer. The oxidized metal iilm of photosurface 14 may be formed on the uncoated portion of the end wall 12, either before, or after the l'electrode mount structure is sealed into the envelope. The method of formation in both cases is similar. It has been found however, that it is an advantage to ,oxidize the metal film after sealing the mount structure `in to the tube as the oxidized metal iilm appears to take ,up water vapor, if it is exposed to the atmosphere bevfore the tube has been sealed.

A iilarnent 58, previously welded between a lead A50 and to the accelerating electrode 16 as shown, is used to form the metal oxide film of photosurface 14. Filament 58 is a ten mil diameter tungsten wire, around which is wrapped a ive mil diameter wire 60 of pure nickel metal. The nickel may be commercial, grade A, nickel having 99.0% minimum of nickel and residual amounts of manganese, silver, sulfur, iron, carbon and copper.

After the glass press 52 has been sealed lto the envelope 10, the exhaust tubulation 56 is connected to `an ex- 'haust system and the ,tube evacuated. All of the tube then is baked-out in an oven at between A260 C. and 280 C. for half an hour, after Which the tube is `cooled to room temperature. In .order to form the photosurface 14, a light source 62 is arranged above the tube end wall 12 and light directed through the envelope onto a photoelectric tube 64 which is connected to an amplifying device 65, having a graduated dial indicating a current iiow proportional to the amount of light from source 62. The indicator 65 can be adjusted to indicate 100 at full transmission of the light through the envelope. While the envelope 10 is still evacuated, a current is passed through a filament 58 to heat and evaporate the nickel wire 60. The levaporated nickel will condense upon the adjacent envelope Wall portion and lform a thin coating on the end wall 12. The nickel lmetal is evaporated until the light transmission from source 62 through the envelope has been reduced to This thickness of nickel is not critical and improved sensitivity can be obtained with layers as thick as 50% transmission or as thin as transmission. Oxygen is then introduced into the bulb through the exhaust tubulation S6, to a pressure of about 700 microns of mercury. The semi-transparent nickel ilm is then oxidized by the use of a high frequency wand placed over the end wall 12. The high frequency potential of the wand produces within the envelope 10 a gaseous discharge which causes the nickel to react with the oxygen in the envelope. The wand is moved over the end wall 12 for about two seconds. This method of oxidizing metal iilms .within an envelope is well known and fully described within U.S. Patent to Essig, 2.020,305. The oxygen within the .envelope is then removed and the reading of indicator 65 is reset to 100.

A deposit of antimony is next put down over the oxidized nickel surface, as follows, by evaporation of antimony in vacuum. A tungsten-molybdenum lament 66 is provided between a lead 57 and the accelerating electrode 16. Fixed to the filament 56 is an antimony pellet 68. The antimony may be high grade commercial antimony having a composition substantially 99.88% antimony and traces of iron, sulfur, arsenic and lead. Passing a current through filament 66 evaporates the antimony pellet and provides a deposit over the oxidized metal iilm on the end wall 12. ri`he evaporation of the antimony is continued until the light transmission through the end wall 12 is approximately 50%. Cesium is then released into the evaporated envelope by heating, with a high frequency coil, a small metal container 70 mounted on a lead wire 59. The cesium permeates through the envelope and condenses on the antimony deposit on the end wall 12. The cesium is evaporated for approximately 7 seconds `to provide a sufficient amount of the metal. The tube is then baked in an oven, at a temperature between C. and 200 C., to promote an activating reaction between the cesium, antimony and oxidized nickel base.

It is diicult to guess, and it is not evident what is the function of the several materials in the photoelectric phenomenon. However, the terms film, layer, coating, and the like, used in the specification are not to be construed as necessarily implying physical continuity and homogeneity. It may be that the materials used form into spaced globules or even molecules and that they may also chemically combine with each other to form a discontinuity not possessed by the usual meaning of the terms used.

A photosurface, made in accordance .with the described method and using an oxidized nickel film, has provided consistently good sensitivity and as high as 80 mcroarnperes per lumen. Pure cobalt metal has also been used to which the cobalt was evaporated and oxi- .dized in the manner described above. Photocathodes made with cobalt have provided, on an average, an output current of 40 microamperes per lumen. Photo- .cathodes have also been effectively made by evaporating a iilm of iron or chromium and oxidizing the iilm. Such photocathodes made vwith iron oxide have produced out- -put currents between 40 and 60 microamperes per lumen, while those made from chromium metal have generally produced output currents of around 27 microamperes per lumen.

Nickel metal is difiicult to put down by evaporation. More success has been obtained in forming a nickel film on the glass support end wall 12 by a sputtering method. Such a method is one in which the metal film is formed on the envelope wall before sealing the electrode mount structure to the envelope 10. A nickel coated metal disk is first mounted within the envelope and the air pressure within the bulb is reduced to substantially 50 microns. The nickel coated disk is used as a cathode and the metal envelope coating 18 as an anode. A gas discharge s established within the tube envelope, such that the positive ions formed bombard the nickel disk and cause the metal of the disk to be sputtered over the inner surface of the envelope 10. Due to the oxygen in the air, the nickel metal comes down on the end wall 12 as nickel oxide. The sputtering is continued until the light transmission through the envelope wall is reduced to between 70% and 95%. An optimum value being around 80% to 90%. The electrode mount structure is then sealed into the envelope 10 and the antimonycesium film deposited on the nickel oxide layer, in the manner described above. The sputtering method may be also used for the other metals, if desired. However the most convenient method is evaporating the metal, if possible, and as described.

It has been found that the amount of oxidation of the metal film tends to control the shift of spectral response from the blue toward the red. Figure 2 discloses several curves relating to photosurfaces made in accordance with my invention. In Figure 2, the relative sensitivity in arbitrary units is plotted against wavelength in A units. Curve 72 represents substantially the sensitivity and spectral response of photosurfaces formed of antimony-cesium deposited upon an unoxidized nickel metal film. Curves 74, 76, and 78 represent photosurfaces formed of antimony and cesium deposited upon a film of oxidized nickel metal, and with increasing oxidation of the nickel metal. It is apparent from the curves, that the oxidation of the metal film provides, first, a significant increase in the sensitivity of the photosurface. Secondly, as the amount of oxidation of the metal takes place, the peak spectral response shifts from the blue into the green and yellow portion of the spectrum.

The novel photosurface has been described above in connection with a photomultiplier tube. However, the use of such a photosurface need not be restricted to this type of tube. The novel photosurface may be used also successfully in a simple phototube, in such applications as colorimetry, where colors are matched, as for example in selecting of colored yarns or in chemical titration. By the process of selective oxidation, as described above, such a photosurface can be peaked at around 5500 A., and by using a blue filter, the response of the photosurface can be adjusted to closely correspond to that of the eye. Such a photosurface would have applications in pick-up tubes for television, such as the Image Orthicon for example, and as disclosed in the copending application of R. E. Johnson,

6 Serial No. 79,328, filed March 3, 1949, now U. S. Patent No. 2,682,479. A red responsive phototube made in accordance to my invention may also be adapted for color pickup tube applications.

From the foregoing it will be apparent that the present nvlention provides an improved antimonycesium cathode and one characterized by its improved sensitivity and color-response characteristic.

What is claimed is:

1. A photosensitive cathode comprising, a transparent supporting base element, an oxidized metal film on said base element, the metal of said film being one from the group consisting of iron, nickel, cobalt, and chromium, a deposit of antimony on said oxidized metal film, and a deposit of cesium on said antimony film.

2. A photosensitive tube comprising, a transparent support member, an anode electrode spaced from one surface of said support member, a photocathode formed on said support member, said photocathode including an oxidized metal film on said one support surface, the metal of said film being one of the group consisting of iron, cobalt, and nickel, a deposit of antimony on said oxidized metal film, and a deposit of cesium on said antimony.

3. A photosensitive cathode comprising, a transparent support base element, an oxidized nickel film on said base element, a deposit of antimony on said oxidized nickel film, and a -deposit of cesium on said antimony film.

4. A photosensitive cathode comprising, a transparent support base element, an oxidized iron film on said base element, a deposit of antimony on said oxidized iron lm, and a deposit of cesium on said antimony film.

5. A photosensitive cathode comprising, a transparent support base element, an oxidized cobalt film on said base element, a deposit of antimony on said oxidized cobalt film, and a deposit of cesium on said antimony film.

6. A photosensitive cathode comprising, a transparent support base element, an oxidized chromium film on said base element, a deposit of antimony on said oxidized chromium film, and a deposit of cesium on said antimony film.

7. A photosensitive tube comprising, a transparent support member, an anode electrode spaced from one surface of said support member, a photocathode formed on said support member, said photocathode including an oxidized nickel film on said one support surface, a deposit of antimony on said oxidized nickel film, and a deposit of cesium on said antimony.

References Cited in the file of this patent UNITED STATES PATENTS 1,991,774 Spencer Feb. 19, 1935 2,242,395 Hartmann et al May 20, 1941 2,40l,736 Janes June ll, 1946 2,506,633 Engstrom May 9, 1950 2,676,282 Polkosky Apt. 20, 1954 

1. A PHOTOSENSITIVE CATHODE COMPRISING, A TRANSPARENT SUPPORTING BASE ELEMENT, AN OXIDIZED FILM ON SAID BASED ELEMENT, THE METAL OF SAID FILM BEING ONE FROM THE GROUP CONSISTING OF IRON, NICKLE, COBALT AND CHROMIUM, A DEPOSIT OF ANTIMONY ON SAID OXIDIZED METAL FILM AND A DEPOSIT OF CESIUM ON SAID ANTIMONY FILM. 